On the oxidation of kraft pulp - Formation of carbonyl groups during acidic hydrogen peroxide oxidation

Licentiate thesis, 2020

A shift from fossil based to bio based raw material is necessary if the needs of a growing population are to be fulfilled sustainably. Today, the packaging industry is heavily dependent on products originating from fossil feedstocks. Lignocellulosic biomass is a well-used material for packaging as its constituents may be used to produce strong and flexible materials. The hydrophilic structure of the cellulose, however, makes utilization of pure cellulose materials in wet or moist conditions difficult since the hydrogen bonds, that are keeping the structure together, are easily disrupted by water. Through the use of additives and mechanical treatments the wet-strength of paper may be slightly increased. An alternative to these techniques is chemical modification of the cellulose itself. By introducing carbonyl groups to the cellulose structure via oxidation, covalent cross-links, i.e. hemiacetals, can form. These cross-links are not interrupted as easily as hydrogen bonds. Traditional methods for introducing carbonyl groups are, however, limited by environmental or economical constraints that prevent them from being suitable for industrial applications.

Oxidative chemistry is typically utilized in the production of bleached qualities of chemical pulp. These processes are optimized to minimize effects on the carbohydrate structures, as this would lead to yield loss or undesirable effects, such as yellowing of the pulp. Altering the conditions in a bleaching stage to favour the introduction of carbonyl groups would allow a carbonyl-rich pulp, with inherent wet-strength, to be produced on a large scale using the existing infrastructure of pulp mills.

The main aim of this work has been to evaluate the oxidation of bleached softwood and hardwood kraft pulp using a common bleaching agent: hydrogen peroxide. Process conditions, such as pH and temperature, have been varied and their influence on the formation of oxidized functionalities and the degradation of the carbohydrates has been analysed. It was found that a pH of 4 greatly increased the formation of carbonyl groups, compared to the traditional alkaline conditions of a hydrogen peroxide stage. Sheets produced from the oxidized pulp showed a significantly enhanced durability at aqueous conditions and were also found to withstand mild stirring in water while exhibiting only limited swelling.